In the 1980s, at peak oil flows, a barrel of oil made the trip from Prudhoe Bay to Valdez in four days.

Now it takes 13 days.

The slower flow causes the temperature of the hot oil to cool faster. At some point, the oil temperature will dip below the freezing point of water along certain segments, unless Alyeska reheats the oil inside the pipe.

As it gets colder, ice and wax may coat the insides of the pipeline. The colder oil might also increase the risk of buried segments of the pipeline jacking up in the ground, company officials said.

The problems have been building for decades and will only become more pressing as oil production declines further.

For example, Alyeska, owned by BP, Conoco Phillips, Exxon Mobil and two smaller companies, used to launch devices to scrape wax — a component of the oil — out of the pipe’s interior every several weeks.